At the present time, commercial copper production is predominantly based on pyrometallurgical methods involving smelting, converting, and electrorefining and to a lesser extent on hydrometallurgical methods involving leaching, solvent extraction, and electrowinning.
One of the serious problems inherent to those pyrometallurgical processes dealing with copper-bearing sulfide minerals is the production of large volumes of fugitive gases, including sulfur dioxide gas. Particularly, the converting and fire refining operations result in the release of large volumes of these gases. The amount of sulfur dioxide gas discharged into the atmosphere has been strictly regulated by both federal and local governments. Sulfur dioxide emission regulations are becoming much tougher, and the industry must either observe the regulations and incur the associated costs, incur formidable and costly penalties, or potentially face forced plant shutdowns.
In order to eliminate or minimize the emission of sulfur dioxide gas, alternative processes have been studied in the past, aimed at convening the sulfide sulfur to sulfate or to elemental sulfur. The route for conversion to sulfate is expensive and may be technically difficult. A more desirable option is to convert sulfides to elemental sulfur, due to the highly marketable value of elemental sulfur. In the past two decades, many researchers tackled this SO.sub.2 problem, focusing on the conversion of sulfide to elemental sulfur. As a result of various endeavors, a number of hydrometallurgical processes were developed, such as the ELECTROSLURRY, Intec, Dextec, and CLEAR processes, in attempts to recover sulfide sulfur in its elemental form.
Envirotech's ELECTROSLURRY process, described in U.S. Pat. Nos. 4,096,053 and 4,066,520, is a sulfate based hydrometallurgical process which is capable of recovering copper from chalcopyrite concentrate, chalcocite concentrates, smelter flue dusts, and cement copper, where sulfides are rejected to the leach residue. In the Envirotech process, leaching and electrowinning are carried out simultaneously in a slurry electrolyte, and catholyte and anolyte are not separated with any kind of diaphragm. This process provides for a simple cell design, high current density electrowinning with no oxygen evolution, and all unit operations under ambient pressure and at temperature below the boiling point. However, in this process, feed materials require a pretreatment to remove any acid soluble impurities and to make the material amenable to be used as a feed for subsequent electrowinning. A substantial weakness of this process is the astoundingly high sulfur content in copper cathode due to the entrapment of sulfides and elemental sulfur.
The Intec Copper Process is a completely hydrometallurgical process using a strong sodium chloride medium where copper sulfide is leached out by cupric ion and air at atmospheric pressure and temperature below 100.degree. C. to solubilize copper as cuprous ion into solution. At the same time, sulfides and iron are rejected into the leach residue as elemental sulfur and goethite, respectively. The leaching is followed by solution purification and high current density electrowinning in a diaphragm cell using dimpled copper sheet as a cathode substrate and RuO.sub.2 /IrO.sub.2 coated titanium mesh as an anode to produce copper granules. The copper granules may require further processing to be in a form acceptable for commercial transactions (i.e. cathode, rod, wire). One of the most undesirable requirements for this process is the use of expensive ion-exchange membranes as the diaphragms for the electrowinning cells. Due to the potentially fast growth of dendrites on the cathode in a chloride electrolyte and due to the high cost and fragility of ion exchange membranes, the electrowinning cells for this process can be very complicated. In addition, leach solutions are highly air sensitive, as oxygen in the air will oxidize cuprous ions. This may result in added cost and complexity in electrowinning cell design and operation.
The Dextec Copper Process, described in U.S. Pat. No. 4,061,552, is similar to the Intec process in some respects. Both processes are chloride based, operate under atmospheric pressure and at temperatures below 100.degree. C., and provide for high current density electrowinning to recover copper as granules or powder. However, in the Dextec Copper Process, both leaching and electrowinning take place simultaneously in a diaphragm cell, with electrolyte purification being carried out after leaching. Chalcopyrite particles are suspended in the slurry anolyte and leached out chemically and/or electrochemically to form cuprous ions, elemental sulfur and ferrous iron, which is further oxidized by oxygen and precipitated as Fe.sub.2 O.sub.3. High chloride concentrations are desirable to stabilize cuprous, and a stream of air bubbles is needed to keep solid particles in suspension and to favor the precipitation of Fe.sub.2 O.sub.3. The slurry anolyte is separated from the catholyte by a polypropylene filter cloth through which clear anolyte rich in cuprous transfers to the catholyte. Copper is recovered on the cathode as copper powder which is removed by the mechanical vibration of the cathode at a frequency of 5 seconds in every 15 minutes. The Dextec Copper Process suffers from a complicated electrowinning cell design and heavy loss of silver. The silver loss is generally attributed to the fact that silver dissolves readily in chloride media and then co-deposits with copper into copper cathode. Further, the Dextec process requires a continuous flow of electrolyte from anode compartment to cathode compartment in order to avoid a substantial decrease in current efficiency. The operating conditions are difficult to control, and the mass balance between the copper depletion in catholyte and copper enrichment in anolyte is difficult to maintain. The fatal problem with the copper product of this process is that it is not adequately pure and may contain unacceptable levels of silver, antimony and bismuth. The product will require further processing to be in a form and purity acceptable for commercial transactions.
Duval's CLEAR process has many common features to the more recent Intec process in that both processes are chloride medium based, both involve multistage leaching with chalcopyrite being converted to cuprous, both result in elemental sulfur and ferric hydroxide precipitation, both have diaphragmed electrowinning cells, and both produce copper granules as a final product. The CLEAR process also has the disadvantage of producing impure copper granules with high silver levels in the copper granules. Another disadvantage of the CLEAR process is the use of an autoclave for its 2nd-stage leaching under a pressure as high as .about.3.5 atm and at a temperature up to 150.degree. C.